1. Field of the Invention
The present invention is directed generally to particle beam measuring instruments, and more specifically to an improved electrode arrangement in a spectrometer objective.
2. Description of the Related Art
A spectrometer objective is known from the publication by E. Plies, entitled "A New Objective Lens with In-Lens Spectrometer for Electron Beam Testing", Proc. XIth International Congress on Electron Microscopy, Kyoto, Japan, Aug. 31-Sept. 7, 1986, pages 625-626. The disclosed spectrometer objective is essentially composed of a magnetic objective lens having a short focal length, a deflection unit arranged within the objective lens, and an retarding field spectrometer which together form the components of an electron-optical column of an electron beam measuring instrument. The disclosed objective lens is used to focus both the primary electrons emitted by a high-current electron source, as well as secondary electrons generated at a specimen by the primary electrons. The secondary electrons are focused by the objective lens to a point lying along the optical axis of the measuring instrument. The focus of the secondary electrons which have been accelerated to high kinetic energies lies in the center of a spherically symmetrical retarding field that is generated by a portion of the spectrometer disposed above the objective lens. The spherically symmetrical retarding field is generated with the assistance of an appropriately formed electrode pair.
To guarantee good focusing of the secondary electrons, one is forced to provide a high positive potential to the extraction electrode disposed immediately above the specimen. High extraction fields (E=1-2 kV/mm) in the region of the specimen, however, should be avoided, particularly when checking the operation of micro-electronic components. In addition, the uniformity of the extraction field is disturbed particularly for highly structured specimens, and this has an unfavorable effect on the topical resolution of the electron beam measuring instrument, which is determined by the diameter of the electron probe on the specimen.